Spartacus Season 1 Episode 13 "Revelations" was the highest rating episode of Season 1 with 1.285 million viewers.
Party Favors (Episode 10) was the previously highest rating episode with 1.265 million viewers. The full List including ratings for each episode can be found on the Spartacus Episode Guide
TVBythenumbers.com reports the following about the ratings:
The penultimate episode of the first season of Starz’ Spartacus: Blood and Sand rebounded from the Good Friday episode with a series high 1.285 million average viewers, edging out the 1.265 million viewers for the March 26 episode.
The 725,000 adults 18-49 was not a series high, but second-best to the 760,000 for the March 26 episode. I calculate the rating right around a .55 so I have rounded it up to .6 to be consistent with the other data in the season one results in the table below.